Recent advances in genetic engineering have provided the prerequisite tools to transform plants to contain alien (often referred to as “heterogenous or heterologous”) or improved endogenous genes. The introduction of such a gene in a plant would desirably lead to an improvement of an already existing pathway in plant tissues or introduction of a novel pathway to modify desired product levels, increase metabolic efficiency, and/or save on energy cost to the cell. It is presently possible to produce plants with unique physiological and biochemical traits and characteristics of high agronomic and crop processing importance. Traits that play an essential role in plant growth and development, crop yield potential and stability, and crop quality and composition are particularly desirable targets for crop plant improvement. These improvements may be achieved by genetically modifying a crop plant for improved carbon assimilation, more efficient carbon storage, and/or increased carbon export and partitioning capabilities.
Atmospheric carbon fixation (photosynthesis) by plants, algae, and photosynthetic bacteria represents the major source of energy to support processes in such organisms. The Calvin cycle, located in the stroma of the chloroplast, is the primary pathway of carbon assimilation in higher plants. Carbon assimilates can either leave the cycle for sucrose or starch biosynthesis or continue through the cycle to regenerate the carbon acceptor molecule, ribulose-1,5-bisphosphate. Sedoheptulose-1,7-bisphosphatase is an enzyme that catalyzes an essentially irreversible reaction in the branch region where intermediates can leave the cycle, and therefore it may be essential to regulating carbon partitioning between the regeneration phase of the cycle and sucrose and starch biosynthesis.
SBPase has no known cytosolic counterpart and is reported to be found only in the chloroplast, where it dephosphorylates sedoheptulose-1,7-bisphosphate (SBP) to form sedoheptulose-7-phosphate and inorganic phosphate. This enzyme is specific for SBP and is inhibited by its products as well as glycerate (Schimkat et al., 1990) and fructose-2,6-bisphosphate (Cadet and Meunier, 1988b). Light, a reducing agent, and Mg2+ are required for activity (Woodrow, 1982; Cadet and Meunier, 1988a). The enzyme is a homodimer with a subunit molecular mass of 35–38 kDa (Nishizawa and Buchanan, 1981; Cadet and Meunier, 1988c).
It has been reported that removal of more than 80% of the enzymatic activity of SBPase in tobacco plants using antisense technology resulted in chlorosis, reduced growth rates, and reduced carbon assimilate levels (Harrison et al., 1998). Reduction in the quantum efficiency of photosystem II was also observed, which resulted in the reduction in carbohydrate content of the leaves. Analysis of carbohydrate status showed a shift from starch while sucrose levels were maintained. These results indicate that SBPase is a potential rate-limiting step in carbohydrate metabolism.
Various sedoheptulose 1,7-bisphosphatases have been characterized biochemically, and the corresponding mRNAs (CDNA) have been cloned from an alga (Genbank accession number: X74418; Hahn and Kuck, 1994) and some higher plants such as Triticum aestivum (Genbank accession number: X65540; Miles et al., 1993), Spinacia oleracea (Genbank accession number: L76556; Martin et al., 1996) and Arabidopsis thaliana (Genbank accession number: S74719; Willingham, et al., 1994). Thus, over-expression of a nucleic acid sequence encoding SBPase in a transgenic plant will provide advantageous results in the plant such as improved carbon assimilation, export and storage; increased photosynthetic capacity; and extended photosynthetic ability.